Remove assets from a stack

Pulls one or more frames out of the stack. The assets themselves are untouched — they remain in the library (and in any albums) and simply appear as individual photos again. IDs that are not current members of the stack are silently ignored.

If a removed frame was the pinned cover, the pin is cleared with no automatic re-pick — clients choose their own display cover. A stack that survives the removal is marked user-owned (origin = user) so burst re-detection honors the edit; a removal that leaves fewer than 2 members dissolves the stack entirely, returning its remaining frames to loose display too. Trashed frames still count as members for that threshold (unlike asset_count, which excludes them), so a stack can survive with asset_count below 2.